Recently, from the viewpoint of increase in environmental consciousness, liquid fuels in which the contents of environmental load substances such as sulfur and aromatic hydrocarbons are small have been demanded. From such a viewpoint, as a technique which can produce a base stock for fuel oil that substantially contains neither sulfur nor aromatic hydrocarbons and is rich in aliphatic hydrocarbons, particularly, a base stock for kerosene and gas oil, a technique called a GTS process (Gas To Liquids) has attracted attention. The GTS process is a method in which synthesis gas containing carbon monoxide gas and hydrogen gas is produced from natural gas by a reforming reaction, this synthesis gas is used as a raw material in a Fischer-Tropsch synthesis reaction (hereinafter, also referred to as the “FT synthesis reaction” in some cases) to produce a hydrocarbon synthetic oil, and the synthetic oil is refined to produce a base stock for fuel oil (see Patent Literature 1, for example).
A synthetic oil obtained by the FT synthesis reaction (hereinafter, also referred to as the “FT synthetic oil” in some cases) is a mixture containing aliphatic hydrocarbons with a wide carbon number distribution as a main component. From this FT synthetic oil, a naphtha fraction containing mainly a component with a boiling point of lower than about 150° C.; middle distillate fraction containing mainly a component with a boiling point of about 150° C. to about 360° C.; and a wax fraction containing mainly a hydrocarbon component heavier than the middle distillate (with a boiling point of higher than about 360° C.) (hereinafter, also referred to as the “FT wax fraction” in some cases) can be obtained.
Among the respective fractions, the middle distillate is the most useful fraction corresponding to a base stock for kerosene and gas oil and is desired to be obtained with a high yield. Therefore, in an upgrading section of hydroprocessing and fractionating the FT synthetic oil to obtain a base stock for fuel oil, the FT wax fraction produced in a significant amount with the middle distillate in the FT synthesis reaction section is converted to a component corresponding to the middle distillate through the hydrocracking to decrease molecular weight, thereby enhancing the yield of the middle distillate as a whole.
The FT wax fraction obtained from the FT synthetic oil by fractionating is hydrocracked in a wax fraction hydrocracking reactor packed with a hydrocracking catalyst, and then separated into gas and liquid in a gas liquid separation apparatus. Then, the liquid component thus obtained (hydrocarbon oil) is sent to a fractionator at the following stage along with the middle distillate preliminarily fractionated from the FT synthetic oil and separately hydrotreated, and the middle distillate (kerosene and gas oil fraction) is obtained by fractionating.
On the other hand, in the FT synthesis reaction, olefins and oxygen-containing compounds containing an oxygen atom derived from carbon monoxide, such as alcohols, are produced as by-products in addition to saturated aliphatic hydrocarbons as a main product, and these by-products (impurities) are contained mainly in the naphtha fraction and the middle distillate obtained by fractionating the FT synthetic oil. Then, when hydrocarbons containing these impurities are used as a fuel, a constituent material for an engine may be damaged and thus these impurities are needed to be removed. This removal of these impurities can be performed by hydrotreating a hydrocarbon oil such as the naphtha fraction and the middle distillate containing the impurities.
Moreover, hydrocarbons produced by the FT synthesis reaction are principally straight-chain aliphatic hydrocarbons, the straight-chain aliphatic hydrocarbons are highly crystalline, and thus a fuel oil containing mainly the straight-chain aliphatic hydrocarbons lacks in cold flow property (fluidity in a low temperature). Therefore, in the middle distillate serving as a base stock for kerosene and gas oil, it is necessary to convert the straight-chain aliphatic hydrocarbons by hydro-isomerizing to branched hydrocarbons to improve the cold flow property. This hydro-isomerizing is generally performed at the same time with hydrotreating.
For a hydrotreating step of hydrotreating including the hydro-isomerizing of the middle distillate, for example, a hydrotreating catalyst is used in which an active metal having hydrogenation activity selected from noble metals belonging to Group 8 to Group 10 in the periodic table is supported by a catalyst support having solid acidity such as zeolite and/or an amorphous composite metal oxide (see Patent Literatures 2 and 3, for example).